The term “cold seal” refers to sealing a package at ambient temperature, typically 15-26° C., using pressure only, as opposed to a high temperature sealant polymer that seals upon the onset of both heat and pressure. Because of their ability to form seals without heat, cold seal cohesives are ideally suited to the packaging of heat sensitive products such as bakery and confectionary products. In addition, cold seal cohesives allow faster packaging speeds to be achieved compared to typical heat-seal packaging methods.
Cold seal cohesives are a class of adhesives that bond primarily to themselves by the application of pressure only. These cohesives produce excellent face-to-face seals and adhere weakly or not at all with face-to-back contact. There are two essential components basic to commercial cold seal cohesives: a cohering component (natural or synthetic rubber) and a reinforcing polymer system that stabilizes and promotes substrate adhesion, and influences the aggressiveness of the cold seal. Natural rubber in latex form is an extremely high molecular weight, branched, and coiled polymer comprised largely of cis-polyisoprene. In a relaxed state, there are numerous molecular entanglements within the polymer chain and with adjoining polymer chains. When two surfaces of natural rubber are brought together, only a minimum of energy is required to induce an interdiffusion with entanglements of surface molecules. This affinity that natural rubber molecules have for themselves characterizes the cohesive bond. Modifying components are added to natural rubber to improve adhesion to the substrate.
The performance of the cold seal can vary based on many factors. Cold seals are perishable and should be used within the manufacturer's suggested shelf life or the performance of the cold seal could be adversely affected. In addition, cold seal cohesives can also stratify in storage and should be stirred prior to use or the cold seal performance could be adversely affected. Also, cold seal cohesives typically have a pH in the 10.0-10.3 range; however, materials that have had a lot of mixing or excessive press time may exhibit a drop in pH. Typically, if the pH drops below 8.5, the cold seal may not apply and perform correctly. High heat and humidity storage of the coated products could adversely affect the performance of the cold seal. Finally, improper drying of the cold seal coating could also negatively affect its sealing performance. As a result of the many variables that may affect the performance of a cold seal cohesive, it is difficult to compare cold seal data using: 1) Different cohesive lots; 2) Different time periods; 3) Different cohesive formulations; 4) Different application equipment; 5) Different coating weights. Therefore it is necessary to compare data tested at the same time, the same cold seal batch, the same application equipment, and the same coating weight. Thus, data gathered in this experiment cannot be directly compared to prior art data.
With most packaging applications, the cold seal is applied in a pattern around the perimeter of the lamination surface. These film laminations typically consist of: 1) An outer print and cold seal release film; 2) A film for direct application of the cold seal; and 3) An adhesive layer to bond these two films together. Typically, the film surface in which the cold seal cohesive is directly applied must have very good adhesion to the cold seal cohesive. If this adhesion of the cohesive to the film substrate does not exist, the consequence will be package failure at the location where the cohesive is in contact with the film substrate. This failure will result in loss of hermetic seal and ultimately spoilage of the food product contained within the package. Cold seal coating weights of 2 to 4.5 lb/ream satisfy most sealing requirements.
Thus, it is necessary to provide a film that has excellent adhesion to the cold seal cohesives. It is also necessary to provide a film that is compliant with US Food and Drug Administration regulations.
U.S. Pat. No. 4,297,187, the disclosure of which is incorporated herein by reference, discloses the use of a discharge treatment method on a plastic surface in a controlled atmosphere of N2 and CO2. This treatment method has been shown to provide a high surface energy to the substrate superior to discharge treatment methods conducted in air or in a purely nitrogenous atmosphere. Such a treated surface is particularly well-suited for metallizing or printing.
U.S. Provisional Patent Application 60/328,410, the disclosure of which is incorporated herein by reference, discloses an oriented, multi-layer film comprising a core layer, a first surface-treated skin layer consisting of a blend of ethylene/propylene copolymer and metallocene polyethylene, or a metallocene propylene homopolymer, or a butene propylene copolymer which provides excellent bond strengths to cold seal cohesives.
U.S. Pat. No. 4,252,851 discloses an oriented, multiple-layer film comprising a core layer and a skin layer comprising a propylene/butene copolymer having 10-15% butene content. The function of this layer is as a heat seal layer and there is no subsequent surface treatment performed on this heat seal layer.
U.S. Pat. No. 5,482,780 discloses a multilayer biaxially oriented polypropylene film having an isotactic polypropylene core, a cold seal release layer on one side of the core and a cold seal receptive layer on the opposite side of the core. The cold seal receptive layer consists of an ethylene-propylene random copolymer containing 2-8 wt % ethylene. This layer can either be flame or corona-treated to improve cold seal receptivity. This patent does not specify another treated layer for metallization on the opposite side of the core. In addition, this patent also uses a different cold seal adhesion formulation than the present invention.
U.S. Pat. No. 5,900,294 discloses a polyolefinic multilayer film comprising a base layer and at least one outer layer comprising a propylene/butene copolymer of 20-30 wt % butene. This patent does mention corona treating one side of the film but does not mention which side of the film is treated or the significance of the corona treatment. In addition, this patent does not mention the use of another treated layer used for metallization.
U.S. Pat. No. 6,022,612 discloses a multilayer biaxially oriented polypropylene film having an isotactic polypropylene polymer core and a cold seal receptive layer on the opposite side of the core. The cold seal receptive layer consists of 30-60% by weight of a thermoplastic rubber and between 40-70% by weight of a polyolefin polymer. The thermoplastic rubber is either a styrene-isoprene-styrene or styrene-butadiene-styrene copolymer. The polyolefin can be virtually any polyolefin from polypropylene, polyethylene, or copolymers thereof. This patent does not specify another treated layer for metallization on the opposite side of the core. In addition, this patent uses a different cold seal adhesion layer formulation than the present invention.
U.S. Pat. No. 6,165,599 discloses an oriented composite film comprised of a polypropylene core, having on one or both surfaces of the core skin layers comprised of a metallocene catalyzed polypropylene and a low molecular weight hydrocarbon resin. The purpose of this skin layer is to improve such properties as the film modulus, haze, and moisture barrier transmission. This patent does not teach the use of the metallocene polypropylene polymer for subsequent surface treatment and cold seal cohesive application.
International Application WO 00/40406 discloses a film structure comprising a first outer layer comprising a migratory additive, a second outer layer comprising a metallocene polyethylene and core layer in between the two outer layers. The metallocene polyethylene outer layer is laminated to a second film. This film does not disclose the subsequent treating or blending of the metallocene polyethylene skin layer. Therefore, the present invention is outside the scope of the above inventions.